Double acting snap action fluidic switch

ABSTRACT

A two position fluid pressure responsive piston has a first effective area exposed to a reference fluid pressure, a larger second effective area exposed to a control fluid pressure, and an intermediate effective area exposed to a pressurized supply fluid from which the control fluid pressure is derived. The control fluid pressure is generated in a fluid passage connecting the pressurized supply fluid with the relatively lower reference fluid pressure and provided with first, second and third series flow restrictions and a variable area valve downstream therefrom and in series flow relationship therewith. A first passage is arranged in parallel with the second and third restrictions and a second passage is arranged in parallel with the third restriction. The control fluid pressure varies in response to opening of the variable area valve causing movement of the piston which, in turn, registers with the first and second passages to sequentially block the same causing a corresponding reduction in the control fluid pressure and a snap action of the piston to one of its two positions. Reverse movement of the piston with a hysteresis effect occurs in response to closing movement of the variable area valve which results in movement of the piston and subsequent opening of the second and first passages to cause snap action of the piston to the other of its two positions.

United States Patent u i McCombs, Jr.

Sept. 3, 1974 DOUBLE ACTING SNAP ACTION FLUIDIC SWITCH lnventor: HowardL. McCombs, Jr., South Bend, 1nd.

The Bendix Corporation, South Bend, 1nd.

Filed: May 21, 1973 Appl. No.: 362,128

[73] Assignee:

[56] References Cited UNITED STATES PATENTS 7/1896 Johnson 91/25 7/1947Cunningham.... 91/47 X 2/1968 Wagner 251/35 X Primary Examiner-ArnoldRosenthal Attorney, Agent, or Firm-Gordon H. Chenez; William N. Antonis[57] ABSTRACT A two position fluid pressure responsive piston has afirst effective area exposed to a reference fluid pressure, a largersecond effective area exposed to a control fluid pressure, and anintermediate effective area exposed to a pressurized supply fluid fromwhich the control fluid pressure is derived. The control fluid pressureis generated in a fluid passage connecting the pressurized supply fluidwith the relatively lower reference fluid pressure and provided withfirst, second and third series flow restrictions and a variable areavalve downstream therefrom and in series flow relationship therewith. Afirst passage is arranged in parallel with the second and thirdrestrictions and a second passage is arranged in parallel with the thirdrestriction. The control fluid pressure varies in response to opening ofthe variable area valve causing movement of the piston which, in turn,registers with the first and second passages to sequentially block thesame causing a corresponding reduction in the control fluid pressure anda snap action of the piston to one of its two positions. Reversemovement of the piston with a hysteresis effect occurs in response toclosing movement of the variable area valve which results in movement ofthe piston and subsequent opening of the second and first passages tocause snap action of the piston to the other of its two positions.

8 Claims, 2 Drawing Figures 3 8 SOURCE 'oF o p 36 PRESSURIZED our urFLU") :I 1 SIGNAL I I 68 2 I 28 l e4 44 70 66 79 I 34 I 42 I Q 50 7 46 Al O Q 58 ,7; 7 74 72 1' as 2 p L4 \}\5 'i .20 Po l 32 A 22 I I A I 12PATENIEBSIP 3 4 was .200

' SOURCE or o 36 PRES$ URlZED- OUTPUT "1 SIGNAL I I0 44 68 82 I a I ase. 34

A I 18 Q 26 I L1 5. As *i 22 OF SNAP ACTION FLOW THROUGH mssness 12amFLOW 'mnousn PASSAGE 72 FLOW 'mnouen o iiYSTERESIS I I I I I I I I 7 FTCLOSED OPEN VALVE 6O POSITION FIG.2

RESTRICTIONS 66,68 a 70 DOUBLE ACTING SNAP ACTION FLUIDIC SWITCHBACKGROUND OF THE INVENTION This invention relates to fluid pressureactuated switching devices of the two position type responsive to acontrol input signal and operative with a hysteresis effect.

Various switching devices of the above-mentioned type are known and usedin control mechanisms to perform an on-off control signal function. Inmany cases the switching device and its associated control circuitry maynot be entirely satisfactory in operation for use in an environmentwhere reliability, accuracy, simplicity, volume, weight and/or costcannot be neglected. One such environment is engine control apparatusfor aircraft engines where maximum reliability and accuracy as well asminimum volume and weight are essential.

SUMMARY OF THE INVENTION It is an object of the present invention toprovide a fluid pressure responsive, multiple position, snap actingswitching device having a characteristic hysteresis effect when movedfrom one position to another.

It is an important object of the present invention to provide a fluidpressure responsive, servo actuated, two position switching devicehaving a hysteresis feature.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 represents a schematic view insection of the present invention.

FIG. 2 represents a series of curves having a P fluid pressure vs servovalve position.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIG. 1, numeral 10designates a casing provided with a chamber 12 having a piston 14slidably carried therein. The piston 14 has a reduced diameter stem orextension 16 which is stepped to provide an annular shoulder 18. Stem 16is slidably carried in an opening 20 formed in a wall 22 separatingchamber 12 from a chamber 24. Chamber 24 is provided with a port 26which communicates via a passage 28 with a low pressure drain fluid P Acompression spring 30 interposed between piston 14 and casing 10preloads piston 14 thereby urging annular shoulder 18 against wall 22which acts as a stop for piston 14. A stop member 32 fixedly secured tocasing 10 at the opposite end of chamber 12 is engaged by piston 14 tolimit movement thereof away from wall 22.

An inlet port 34 is connected via a passage 36 to a source ofpressurized fluid at pressure P The source of pressurized fluid fromwhich pressures P and P are derived is generally indicated by 38 and isconventional in that it may include fluid pump means, not shown,supplied fluid at relatively low inlet pressure P and adapted todischarge fluid at a higher pressure P which may be regulated to aconstant value if desired.

A passage 40 connects inlet port 34 to a port 42 which, in turn,communicates via a passage 44 with an inlet port 46 in a casing 48 whichmay be remotely located from casing 10. The port 42 is vented via apassage 50 to a chamber 52 in casing 48 which chamber 52 is providedwith a port 54 connected via a passage 56 to passage 28 at pressure P Alever 58 pivotally mounted at one end to casing 48 is provided with aservo or flapper valve 60 adapted to coact with thedischarge end ofpassage 50 to thereby vary the effective flow area thereof. The lever 58is actuated by a rotatable cam 62 suitably disposed in chamber 52 andactuated by a control lever 64.

A plurality of fluid flow restrictions 66, 68 and 70 are disposed inpassage 40 upstream from valve 60 and in series flow relationshiptherewith. A branch passage 72 connects passage 40 intermediaterestrictions 66 and 68 with chamber 12. A branch passage 74 connectspassage 40 intermediate restrictions 68 and 70 with chamber 12. A branchpassage 76 connects passage 40 intermediate restriction 70 and valve 60with chamber 12. The passage 40 upstream from restriction 66 is ventedvia a passage 78 to chamber 12 thereby communicating fluid at pressure Pto an intermediate effective area side A of piston 14 which intermediatearea side is equivalent to the effective area A on the opposite side ofpiston 14 exposed to pressure P minus thev end area A of stem 16 exposedto pressure P An outlet port 79 is connected via a passage 80 to chamber12 which passage 80 is blocked as shown by piston 14 in the positionshown in FIG. 1. However, piston 14 may move, in a manner to bedescribed, away from wall 22 to unblock passage 80 thereby ventingsupply fluid at pressure P from passage 78 to outlet port 79 which, inturn, may be connected via a passage 82 to fluid pressure responsivecontrol means, not shown.

It will be noted that, in view of the ratio relationship of the areas AA and A of piston 14, a force balance will exist on piston 14 at apressure P which has a predetermined percentage relationship relative tothe pressures P and P opposing the same. Any deviation of pressure P Xfrom the predetermined percentage relationship will unbalance the piston14 in one direction or the other depending upon the relative error inpressure P It will be assumed that the network shown in FIG. 1 ispressurized with fluid at the respective pressures P P and P The controllever occupies a position whereby flapper valve 60 is substantiallyclosed to generate a pressure P upstream therefrom which unbalancespiston 14 to the position shown. Movement of lever in a clockwisedirection from the position shown results in opening movement of flappervalve 60 and a resulting drop in pressure P in chamber 12 as indicatedby curve A from point 1 to point 2 in FIG. 2. Upon reaching apredetermined position and thus area of flapper valve 60 wherebypressure P X which is a predetermined percentage of pressures P and Popposing the same, a force unbalance on piston 14 is generated inresponse to which the piston 14 moves away from wall 22 thereby chokingflow from passage 72 to chamber 12 which causes a correspondingreduction in :pressure P in chamber 12 thereby tending to hasten themovement of piston 14 and subsequent blocking of ,passage 72. Blockingof passage 72 and the resulting sudden decrease in pressure P in chamber12 causes piston 14 to move with a snap action against stop 32 asindicated by curve A from point 2 to point 3. As the piston 14 movesinto engagement with stop 32, it slides past passage 74 to block thesame causing a further reduction in pressure P thereby producing apositive holding action of piston 14 against stop 32. It will be notedthat, with passages 72 and 74 blocked, a'll flow to chamber 12 isthrough series restrictions 66, 68 and 70 which establishes acorresponding pressure P vs. flapper valve 60 position relationship asshown by curve B. Referring to passage 80, it will be noted that therear side of piston 14 exposed to pressure P approaches passage 80 aspassage 72 is blocked such that substantially simultaneously withinitiating the snap action of piston 14 the passage 80 is fully openedby piston 14 to vent pressure P to output port 79 and thus passage 82 totrigger the control means, not shown, connected thereto.

The piston 14 will remain seated against stop 32 until the control lever64 is actuated to reset flapper valve 60 in a closing direction toeffect a corresponding increase in pressure P as shown in FIG. 2 frompoint 3 to point 4 of curve B. At point 4 of curve B, a pressure Pequivalent to that at point 2 on curve A is generated in response towhich the piston 14 is unbalanced away from stop 32. The difference inposition of flapper valve 60 at which the same pressure P is generatedas represented by the spread between points 2 and 4 in FIG. 2 isindicative of desired hysteresis derived from the added restriction toflow caused by blocking of passages 72 and 74 which hysteresis is forthe purpose of preventing cycling of the piston 14 regardless of theposition of flapper valve 60.

As the piston 14 moves away from stop 32 and traverses passage 74 toprogressively open the same, the pressure P increases rapidly causingthe piston 14 to react with a snap action into engagement with wall 22as indicated by curve B from point 4 to point 5. It will be noted thatthe snap action of piston 14 occurs as the front side of piston 14exposed to pressure P approaches passage 80 such that the passage 80 isblocked substantially instantaneously by piston 14 to depressurizeoutput port 79 as well as the control means, not shown, responsivethereto. Movement of piston 14 from passage 74 into engagement with wall22 results in opening of passage 72'which has the effect of increasingpressure P X accordingly to provide a positive holding action bypressure P of piston 14 against wall 22.

It will be recognized that the output pressure signal P derived frompassage 80 is only one form of output signal which may be controlledbypiston 14. If desired the passage 80 and output port 79 may beeliminated and suitable linkage means connected to piston 14 to providea position output signal depending upon the type of control desired.

The restrictions 66, 68 and 70 may be selected with equal areas or withvarious area relationships depending upon the particular controlcharacteristics desired. It will be recognized that the relative slopeof curves A and B as well as intermediate curve C may be adjusted bysuitable variation of the effective flow area of one or more of therestrictions 66, 68 and 70.

The spring 30 is provided to impose a relatively small force preloadagainst piston 14 thereby holding the same against wall 22 in theabsence of pressurization of piston 14.

, Iclaim:

l. Fluid pressure responsive control apparatus comprising:

a supply source of pressurized fluid;

a drain source of pressurized fluid;

conduit means connecting said supply source and drain source includingfirst, second and third flow restrictions in series flow relationshipand a variable area valve downstream therefrom and in series flowtherewith for generating a control fluid pressure which varies inresponse to the position of said 5 variable area valve;

first flow passage means in series flow with said first restriction andparallel flow with said second and third restrictions;

second flow passage means in series flow with said first and secondrestrictions and parallel flow with said third restriction;

fluid pressure differential responsive means responsive to said controlfluid pressure and said supply fluid pressure and operatively connectedto said first and second flow passage means for controlling fluid flowtherethrough depending upon the position of said pressure differentialresponsive means in response to a predetermined differential betweensaid control and supply pressures;

said pressure differential responsive means being actuated to a firstposition in response to a first position of said variable area valvewhereby said first and second flow passage means are opened;

said pressure difierential responsive means being actuated to a secondposition in response to a second position of said variable area valvewhereby said first and second flow passage means are blocked; and

signal producing means operatively connected to said pressuredifferential responsive means.

2. Fluid pressure responsive control apparatus as claimed in claim 1wherein:

said pressure differential responsive means is a piston provided withopposite faces exposed to said control fluid pressure and supplypressure and adapted to slidably traverse said first and second flowpassage means to vary the flow therethrough in sequence. 3. Fluidpressure responsive control apparatus as 0 claimed in claim 2 andfurther including:

a first casing having a portion of said conduit means including saidfirst, second and third restrictions contained therein;

a chamber in said casing slidably containing said piston;

a second casing having a spaced-apart relationship relative to saidfirst casing and a portion of said conduit means including said variablearea valve contained therein; and

fluid passage means defining a portion of said conduit meansinterconnecting said first and second casings.

4. Fluid pressure responsive control apparatus as claimed in claim 1wherein:

said first, second and third flow restrictions have fixed flow areas.

5. Fluid pressure responsive control apparatus as claimed in claim 2 andfurther including:

said first and second positions of said piston being defined by fixedspaced-apart stop means adapted to be engaged by said piston.

6. Fluid pressure responsive control apparatus as claimed in claim 2wherein:

said piston is unbalanced from said first position by thereby causing arapid drop in said control fluid pressure and corresponding increase inforce unbalance imposed on said piston resulting in snap action movementof said piston into engagement with said stop means defining said secondposition;

said second flow passage means being traversed and closed by said pistonto produce a further decrease in said control fluid pressure therebyincreasing the force unbalance imposed on said piston tending to holdthe same in said second position.

claimed in claim 1 wherein:

said signal producing means includes third flow passage means connectedto said conduit means up- 7. Fluid pressure responsive control apparatusas claimed in claim 6 wherein:

said piston is unbalanced from said second position stream from saidfirst, second and third flow restrictions for providing an outputpressure signal;

said fluid pressure differential responsive means by an increase in saidcontrol fluid pressure and movable in response to the resulting forceunbalbemg operatlvely connecfted to 531d third flow P ance imposedthereon in a direction to open said g means for comfcfllmgtherethl'olfgh second passage thereby causing a rapid increase inpending upon the position of said pressure differenforce balance imposedon said piston resulting in tial responsive means. snap action movementof said piston into engage-

1. Fluid pressure responsive control apparatus comprising: a supplysource of pressurized fluid; a drain source of pressurized fluid;conduit meanS connecting said supply source and drain source includingfirst, second and third flow restrictions in series flow relationshipand a variable area valve downstream therefrom and in series flowtherewith for generating a control fluid pressure which varies inresponse to the position of said variable area valve; first flow passagemeans in series flow with said first restriction and parallel flow withsaid second and third restrictions; second flow passage means in seriesflow with said first and second restrictions and parallel flow with saidthird restriction; fluid pressure differential responsive meansresponsive to said control fluid pressure and said supply fluid pressureand operatively connected to said first and second flow passage meansfor controlling fluid flow therethrough depending upon the position ofsaid pressure differential responsive means in response to apredetermined differential between said control and supply pressures;said pressure differential responsive means being actuated to a firstposition in response to a first position of said variable area valvewhereby said first and second flow passage means are opened; saidpressure differential responsive means being actuated to a secondposition in response to a second position of said variable area valvewhereby said first and second flow passage means are blocked; and signalproducing means operatively connected to said pressure differentialresponsive means.
 2. Fluid pressure responsive control apparatus asclaimed in claim 1 wherein: said pressure differential responsive meansis a piston provided with opposite faces exposed to said control fluidpressure and supply pressure and adapted to slidably traverse said firstand second flow passage means to vary the flow therethrough in sequence.3. Fluid pressure responsive control apparatus as claimed in claim 2 andfurther including: a first casing having a portion of said conduit meansincluding said first, second and third restrictions contained therein; achamber in said casing slidably containing said piston; a second casinghaving a spaced-apart relationship relative to said first casing and aportion of said conduit means including said variable area valvecontained therein; and fluid passage means defining a portion of saidconduit means interconnecting said first and second casings.
 4. Fluidpressure responsive control apparatus as claimed in claim 1 wherein:said first, second and third flow restrictions have fixed flow areas. 5.Fluid pressure responsive control apparatus as claimed in claim 2 andfurther including: said first and second positions of said piston beingdefined by fixed spaced-apart stop means adapted to be engaged by saidpiston.
 6. Fluid pressure responsive control apparatus as claimed inclaim 2 wherein: said piston is unbalanced from said first position by adecrease in said control fluid pressure and movable in response to theforce unbalance imposed thereon in a direction to close said firstpassage thereby causing a rapid drop in said control fluid pressure andcorresponding increase in force unbalance imposed on said pistonresulting in snap action movement of said piston into engagement withsaid stop means defining said second position; said second flow passagemeans being traversed and closed by said piston to produce a furtherdecrease in said control fluid pressure thereby increasing the forceunbalance imposed on said piston tending to hold the same in said secondposition.
 7. Fluid pressure responsive control apparatus as claimed inclaim 6 wherein: said piston is unbalanced from said second position byan increase in said control fluid pressure and movable in response tothe resulting force unbalance imposed thereon in a direction to opensaid second passage thereby causing a rapid increase in force balanceimposed on said piston resulting in snap action movement of said pistoninto engagement with said stop means defining said first position; saidfirst flow passage means being traversed and opened by said piston toproduce a further increase in said control fluid pressure therebyincreasing the force unbalance imposed on said piston tending to holdthe same in said second position.
 8. Fluid pressure responsive controlapparatus as claimed in claim 1 wherein: said signal producing meansincludes third flow passage means connected to said conduit meansupstream from said first, second and third flow restrictions forproviding an output pressure signal; said fluid pressure differentialresponsive means being operatively connected to said third flow passagemeans for controlling flow therethrough depending upon the position ofsaid pressure differential responsive means.